Operations for natural gas processing can range from simple treating and conditioning for pipeline delivery to complex operations needed to meet stringent specifications to produce pipeline sales gas or liquefied natural gas (LNG). The removal of acid gas compounds and other impurities such as CO2, H2S, COS, organic sulfur compounds, water and mercury is required to meet end product specifications and to avoid product blockages in the downstream process equipment. Such undesired blockages include frozen carbon dioxide and water. The level of treatment varies according to the treated gas product specifications and can vary according to local environmental regulations.
In general, mercury is removed by a non-regenerable guard bed and carbon dioxide and hydrogen sulfide are removed by a solvent process such as an amine process. Water is removed by a molecular sieve dehydration unit. A major challenge is to remove COS and organic sulfur compounds such as mercaptan, disulfide, and polysulfide. Historically, there are two types of flow schemes to remove these organic sulfur compounds to meet the product requirements. One type is gas-phase treatment, where the molecular sieve unit is designed to remove organic sulfur compounds that then pass into a regeneration gas stream. The regeneration gas can then be treated by a physical solvent unit such as that used by the Selexol™ process sold by UOP LLC to produce a sales or fuel gas and an acid gas stream containing the organic sulfur compounds. This acid gas stream, along with the acid gas generated in the amine unit, can be sent to a sulfur plant such as Claus process for sulfur recovery. The other scheme that is known is a liquid phase treatment, where all or most of the COS or organic compounds are allowed to slip through the dehydration unit. If a natural gas liquid (NGL) unit is in place, these sulfur compounds are expected to be concentrated in the NGL stream. The NGL liquids or after fractionation are then treated specifically for COS removal by a specialty amine unit. Mercaptans are then removed by a regenerable caustic process such as the Merox process from UOP LLC. Finally, the liquid is sulfur-polished by a molecular sieve unit to reduce the sulfur level to the required specification.
Both of these prior art processes require substantial capital and operational investments. The gas phase treatment option requires a sulfur plant. The liquid phase option requires an NGL unit. The liquid phase scheme also has an unattractive element in the caustic-based treatment and its associated spent caustic disposal problem. The sulfur plant is quite costly and is only justified when the sulfur level is sufficiently high. The use of an NGL plant is only justified when the gas is rich in C2, C3 and C4 components. If the gas processing operator receives their feed gas from different supply sources, the feed gas sulfur level and/or its hydrocarbon contents may vary from time to time, which would not justify investment in a sulfur plant or the continuous operation of the NGL unit. Under this circumstance, an alternative sulfur capture technology is needed. The ideal solution is to remove the sulfur compounds from the natural gas stream and to turn these sulfur compounds into either a solid or a liquid form so that they can be physically transported out of the gas processing facility.